Certain communication terminals (possibly of the satellite type), for example those, where applicable for collective use, that are installed in transportation means such as motor vehicles (buses, coaches, trucks, cars), trains, aircraft or boats, have send/receive antennas the dimensions whereof can cause problems in the presence of certain utilization constraints.
Thus to enable operation in the Ku band (12 to 18 GHz), it has been proposed that the send/receive antennas have a dimension along their greater axis between 60 and 70 cm. At the same time, international standardization organizations such as the ETSI (European Telecommunication Standards Institute) and the FCC (Federal Communication Commission) are imposing limitations on the off-axis power spectral density to adjacent satellites in order to limit the level of interference they receive. This limitation is defined by conditions involving the off-axis angle (θ) of the main lobe of the antenna and the ratio (K) between the aggregate power density of all the terminals transmitting at the same time on the same frequency band (full load) and the power density of a single communication terminal. Remember that in the case of MF-TDMA type transmission, K is equal to 1, in contrast to the case of CDMA type transmission in which K is the number of simultaneous users.
It can be shown that under certain conditions, for example in the presence of DVB-RCS (Digital Video Broadcasting—Return Channel by Satellite) satellite communications, a communication terminal equipped with a 60 to 70 cm antenna has an off-axis power spectral density higher than that authorized by the conditions defining the limitation referred to above. It is then necessary to reduce the power spectral density used by a factor that can be equal to 10.
A standard solution for reducing the power spectral density is to use a direct spreading technique that multiplies the wanted signal by a sequence, for example of pseudo-random type, with a symbol timing rate (or chip timing rate) higher than the payload symbol timing rate. Unfortunately this solution has at least three drawbacks: it necessitates modification of the communication terminal so that it is able to effect the aforementioned multiplication, it necessitates a modification of the demodulator, which is installed in a communication gateway (possibly of the satellite type), for example, in order for it to be able to decode the new symbol timing, and it necessitates on reception synchronization of the codes with a view to despreading the signal, which requires a temporal accuracy of the order of a fraction of a symbol (or chip).
Another solution uses FEC type codes with a 1/N redundancy higher than that (equal to 1/2) of the lowest code of the DVB-RCS standard. This solution reduces the Es/NO ratio (energy of one symbol/noise power spectral density) and consequently the power spectral density. Unfortunately, this solution also has at least three drawbacks: it necessitates modification of the communication terminal so that it is able to use a symbol timing rate lower than that of the DVB-RCS standard, it necessitates modification of the demodulator, which is installed in a communication gateway (possibly of the satellite type), for example, for it to be able to decode the new coding timing rates, and it leads to a probable limitation of the Es/NO ratio because of the receive demodulation threshold, and consequently modification of the algorithms of the demodulator in order to improve that threshold.